The Bayer process for the production of alumina necessary for aluminum production depends upon both the yield and particle size of alumina produced in the process. The process is also dependent upon the bauxite digestion, clarification for the removal of the production residue, the extraction of hydrate alumina from this clarified liquor using a precipitation process, separation of the finer hydrate precipitation solids for eventual seeding operations and finally washing and drying of the coarser solids obtained from the precipitation process for eventual calcination into the final anhydrous alumina.
Production of alumina from this process is greatly increased if the precipitation operation is optimized. This precipitation is induced by the injection of seed material which is generally alumina trihydrate particles. Dissolved alumina in the clarified process stream precipitates by crystallization upon the surfaces of the injected seed material. Therefore, control of the amount of injected seed material added at this precipitation step is very critical to the entire operation. Injection of too much seed into the precipitation step will cause insufficient crystal growth upon the seed and will reduce the amount of product having a quality size. Injection of too little seed material will also reduce the amount of product and will reduce the particle quality of any alumina precipitated out in the process. In determining the optimum amount of seed material to be injected into the precipitation step of the Bayer process, the total surface area of the seed material should be accurately measured since the precipitation takes place most efficiently on the seed material's available surface area.
Previous methods of controlling the amount of seed material added in the Bayer process have generally been based upon controlling the amount of seed material added to a given volume of process liquid. This amount has been measured by determining the amount of seed material removed from storage tanks, by using level measurement or by calculating the volume injected using a flow meter or a mass flow meter. The problem with these methods is that the surface area of the injected seed material may have changed during the process, requiring adjustments in the amount of seed material injected.
Other methods of determining the surface area of seed material have been contemplated, particularly in "The Chemical Background of the Aluminum Industry", by T. G. Pearson, The Royal Institute of Chemistry, 1955, Page 30. This particular reference mentions that such techniques as air permeability, light extinction or gas absorption may be utilized for determining the surface area of injected seed material. There is no detailed discussion in this reference as to how these techniques may be utilized.
Therefore, it is a primary object of the present invention to provide a method for determining the surface area of injected seed material in the Bayer process for the production of alumina.
It is another object of the present invention to provide a method as above which provides control and accuracy on a continuous basis.
It is a further object of the present invention to provide a method as above which directly controls the amount of injected seed material in the Bayer process based upon the amount of desired surface area of the seed material.
Further objects and advantages of the present invention will become apparent from a consideration of the following description.